The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Message Formats and Message Processing: RFC 822 and MIME                           9  TCP/IP Enhanced Electronic Mail Message Format: Multipurpose Internet Mail Extensions (MIME)

MIME Content-Type Header and Discrete Media: Types, Subtypes and Parameters 1 2 3 4 5 6 MIME Content-Transfer-Encoding Header and Encoding Methods

Multipart Message Encoding

You can see just from these different subtypes how much flexibility the multipart type provides to MIME, and there are others. In all cases, the same syntax is used to encode the constituent body parts into a single message. The basic process is as follows:

1. Each individual piece of data is processed as if it were to be transmitted as the body of a discrete media type MIME message. This includes the specification of appropriate headers, such as Content-Type, Content-ID and Content-Transfer-Encoding, as needed.
   
   
2. A special boundary delimiter is chosen to separate the body parts. It must be selected so that it will not appear in any of the body parts; a random string is sometimes used. It is prepended with two dashes (“--”) when placed in the message to reduce the chance of it being mistaken for data.
   
   
3. The multipart message is assembled. It consists of a preamble text area, then a boundary line, followed by the first body part. Each subsequent body part is separated from the previous one with another boundary line. After the last body part, another boundary line appears, followed by an epilogue text area.
   
   
4. The special parameter boundary is included in the Content-Type header of the message as a whole, to tell the recipient what pattern separates the body parts.

MIME Content-Type Header and Discrete Media: Types, Subtypes and Parameters 1 2 3 4 5 6 MIME Content-Transfer-Encoding Header and Encoding Methods